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Asymmetric induction and photochemistry of 7-benzoylbicyclo[2.2.1]heptane derivatives Scott, Charles James


The Norrish/Yang photochemistry of two series of norbornane derivatives, 7-methyl-7- benzoylnorbornanes and 7-benzoylbenzonorbornenes, has been studied in both the solid state and solution. In the 7-benzoylbenzonorbornene system, photolysis in the solid state leads to a significant alteration in the ratio of photoproducts, from a complex mixture of two Yang cyclobutanols and a Norrish type II cleavage product in solution, to a single product, an endo-aryl cyclobutanol, in the solid state due to the confining effects of the crystal lattice. Photolysis of the 7-methyl-7-benzoylnorbornanes in the solid state or solution led to formation of an endo-aryl cyclobutanol as the sole photoproduct. By utilizing ionic chiral auxiliaries it was possible to form chiral ammonium carboxylate salts between optically pure amines and the achiral norbornane derivatives containing a carboxylic acid functional group. Photolysis of the chiral salts in the solid state gave variable results, but through the use of a number of auxiliaries it was possible to achieve a high degree of enantioselectivity in the photoproducts (up to 98% ee at 100% conversion of the starting ketone). Photolysis of the same salts in solution gave only a racemic mixture of photoproducts, highlighting the critical role that the chiral crystal lattice plays in the asymmetric induction. Through the use of X-ray crystallography, solid state reactivity-crystal structure relationships were developed to explain the observed reactivity in 11 of the ketone substrates. Fortuitously, two of the molecules studied underwent single crystal-to-single crystal reactions, allowing for a detailed study of the reaction through a series of X-ray crystal structures. In this case, because the molecules studied were chiral ammonium carboxylate salts used in the asymmetric induction studies, it was possible to predict and confirm the absolute configuration of the photoproduct, as well as validate the crystal structure-reactivity relationships. Comparison of the results obtained in the studies presented have also been compared to previous work in order to develop a greater understanding of how conformational changes in the geometry of a ketone can affect the outcome of Norrish/Yang reactions.

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